Systems and methods to enhance spatial diversity in distributed-input distributed-output wireless systems

ABSTRACT

Systems and methods are described for enhancing the channel spatial diversity in a multiple antenna system (MAS) with multi-user (MU) transmissions (“MU-MAS”), by exploiting channel selectivity indicators. The proposed methods are: i) antenna selection; ii) user selection; iii) transmit power balancing. All three methods, or any combination of those, are shown to provide significant performance gains in DIDO systems in practical propagation scenarios.

RELATED APPLICATIONS

This application is a continuation of the following co-pending U.S.patent application Ser. No. 13/475,598, filed May 18, 2012, which is acontinuation-in-part of the following co-pending U.S. patentapplications and issued patents:

U.S. application Ser. No. 13/464,648, entitled “System and Methods toCompensate for Doppler Effects in Distributed-Input Distributed OutputSystems.”

U.S. application Ser. No. 12/917,257, entitled “Systems And Methods ToCoordinate Transmissions In Distributed Wireless Systems Via UserClustering”

U.S. application Ser. No. 12/802,988, entitled “Interference Management,Handoff, Power Control And Link Adaptation In Distributed-InputDistributed-Output (DIDO) Communication Systems”

U.S. Pat. No. 8,170,081, issued May 1, 2012, entitled “System And MethodFor Adjusting DIDO Interference Cancellation Based On Signal StrengthMeasurements”

U.S. application Ser. No. 12/802,974, entitled “System And Method ForManaging Inter-Cluster Handoff Of Clients Which Traverse Multiple DIDOClusters”

U.S. application Ser. No. 12/802,989, entitled “System And Method ForManaging Handoff Of A Client Between DifferentDistributed-Input-Distributed-Output (DIDO) Networks Based On DetectedVelocity Of The Client”

U.S. application Ser. No. 12/802,958, entitled “System And Method ForPower Control And Antenna Grouping In ADistributed-Input-Distributed-Output (DIDO) Network”

U.S. application Ser. No. 12/802,975, entitled “System And Method ForLink adaptation In DIDO Multicarrier Systems”

U.S. application Ser. No. 12/802,938, entitled “System And Method ForDIDO Precoding Interpolation In Multicarrier Systems”

U.S. application Ser. No. 12/630,627, entitled “System and Method ForDistributed Antenna Wireless Communications”

U.S. Pat. No. 7,599,420, issued Oct. 6, 2009, entitled “System andMethod for Distributed Input Distributed Output Wireless Communication”;

U.S. Pat. No. 7,633,994, issued Dec. 15, 2009, entitled “System andMethod for Distributed Input Distributed Output Wireless Communication”;

U.S. Pat. No. 7,636,381, issued Dec. 22, 2009, entitled “System andMethod for Distributed Input Distributed Output Wireless Communication”;

U.S. Pat. No. 8,160,121, issued Apr. 17, 2012, entitled, “System andMethod For Distributed Input-Distributed Output WirelessCommunications”;

U.S. application Ser. No. 11/256,478, entitled “System and Method ForSpatial-Multiplexed Tropospheric Scatter Communications”;

U.S. Pat. No. 7,418,053, issued Aug. 26, 2008, entitled “System andMethod for Distributed Input Distributed Output Wireless Communication”;

U.S. application Ser. No. 10/817,731, entitled “System and Method ForEnhancing Near Vertical Incidence Skywave (“NVIS”) Communication UsingSpace-Time Coding.”

BACKGROUND

Prior art multi-user wireless systems add complexity and introducelimitations to wireless networks which result in a situation where agiven user's experience (e.g. available bandwidth, latency,predictability, reliability) is impacted by the utilization of thespectrum by other users in the area. Given the increasing demands foraggregate bandwidth within wireless spectrum shared by multiple users,and the increasing growth of applications that can rely upon multi-userwireless network reliability, predictability and low latency for a givenuser, it is apparent that prior art multi-user wireless technologysuffers from many limitations. Indeed, with the limited availability ofspectrum suitable for particular types of wireless communications (e.g.at wavelengths that are efficient in penetrating building walls), priorart wireless techniques will be insufficient to meet the increasingdemands for bandwidth that is reliable, predictable and low-latency.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent publication with color drawing(s)will be provided by the U.S. Patent and Trademark Office upon requestand payment of the necessary fee.

A better understanding of the present invention can be obtained from thefollowing detailed description in conjunction with the drawings, inwhich:

FIG. 1 illustrates one embodiment of a multi-user (MU) multiple antennasystem (MAS), or MU-MAS, consisting of a precoding transformation unit.

FIG. 2 illustrates one embodiment in which base transceiver stations(BTSs) are directly connected to a centralized processor (CP).

FIG. 3 illustrates an embodiment in which base transceiver stations(BTSs) are connected via a base station network.

FIG. 4 illustrates another embodiment in which base transceiver stations(BTSs) are connected via a network.

FIG. 5 illustrates simulated transmit covariance matrices for DIDO 6×6systems with three models.

FIG. 6 illustrates the cumulative density function (CDF) of thediversity metric as well as the symbol error rate (SER) performance as afunction of the signal-to-noise ratio (SNR) for three channel models.

FIG. 7 illustrates an exemplary distribution of base transceiverstations.

FIG. 8 illustrates space selectivity indicator (SSI) as a function ofthe maximum auto-correlation and cross-correlation coefficients of thespatial covariance matrix.

FIGS. 9A-9C illustrate exemplary SSI results for three channel models.

FIG. 10 illustrates a comparison of the cumulative density function(CDF) of the SSI in the three scenarios above.

FIG. 11 illustrates the SNDR of client devices in a DIDO 2×2 system forone measurement set.

FIG. 12 illustrates combined plots containing SNDR, TSI, and SSI.

FIG. 13 illustrates the results from a second channel scenario where RX2moves from a location with high power imbalance to another with lowpower imbalance.

FIG. 14 illustrates that even the performance of RX1 (the stationaryclient) improves as the SSI increases.

FIGS. 15A-15B illustrate the SNDR versus the average SSI (a) andstandard deviation of the SSI (b).

FIG. 16 illustrates how SNDR decreases as the standard deviation of theTSI due to deep-fade in time caused by a client's mobility and Dopplereffect.

FIG. 17A illustrates the CN defined in equation (7) plotted as afunction of the minimum auto-correlation coefficient and maximumcross-correlation coefficient, where every dot is the result of 100 msecof data.

FIG. 17B illustrates the CDF of the SSI defined as the CN.

FIG. 18 illustrates a three dimensional version of FIG. 17 a.

FIG. 19 illustrates the average SNDR as a function of the average CN.

FIGS. 20A-20B illustrate the performance of different order DIDO systemsin i.i.d. channels assuming same per-antenna transmit power.

FIGS. 21A-21B illustrate the simulated performance of DIDO 4×4 in i.i.d.channels.

FIGS. 22A-22B illustrate the gain in SNR as a function of the gain ofthe diversity metric in (9).

FIGS. 23A-23B illustrate the traces of SNDR, SSI (indicated as λ_(min))and TSI (being the absolute value of the complex channel gain from everytransmitter indicated as TX1, . . . , M).

FIGS. 24A-24B illustrate the results obtained over the same route as inFIG. 23, but using two extra antennas.

FIGS. 25A-25B, 26A-26B, 27A-27B, and 28A-28B show the simulated SERperformance and CDF of the SSI for DIDO 4×2, 6×4, 8×6 and 10×8 systems.

FIGS. 29A-29B illustrates the average SNDR (over periods of 5 seconds)versus the average SSI.

FIGS. 30A-30B illustrates results for DIDO 4×4 and 6×4.

FIGS. 31A-31B illustrates the average SNDR as a function of the standarddeviation of the TSI for DIDO 2×2 and 4×2.

FIGS. 32A-32B illustrates results for DIDO 4×4 and 6×4.

FIG. 33 compares the CDF of the instantaneous SSI for four DIDO systems.

FIG. 34 illustrates the results from one particular measurement set withapproximately 20 seconds of data.

FIG. 35 illustrates one embodiment where the SSI target is raised to −15dB and as a result the SER performance improves.

FIG. 36 illustrates improvements achieved by raising the target SSI.

FIG. 37 illustrates an exemplary scenario where multiple UEs areclustered around one BTS.

FIG. 38 illustrates SER performance of different order DIDO systems fordifferent values of the maximum auto-correlation coefficient.

FIG. 39 illustrates the CDF of the SSI for different values of maximumauto-correlation.

FIG. 40 illustrates one embodiment of a method for balancing thetransmit power across all BTSs in the MU-MAS or DIDO system.

FIG. 41 illustrates another embodiment of a method for balancing thetransmit power across all BTSs in the MU-MAS or DIDO system.

FIG. 42 illustrates the performance of the transmit power balancingmethods in practical outdoor propagation scenarios.

FIG. 43 illustrates the distribution of the condition number with andwithout power imbalance.

FIGS. 44-46 illustrate the channel traces (SNDR, SSI and TSI) for threedifferent cases: i) DIDO 2×2 without transmit power balancing method;ii) DIDO 2×2 with transmit power balancing method; iii) DIDO 4×2 withtransmit power balancing method in combination with antenna selectionmethod.

FIG. 47 illustrates a scenario where a particular BTS is the source oftransmit power imbalance.

FIG. 48 illustrates the condition number distribution with and withoutimbalance.

FIGS. 49-51 depict channel traces for different algorithms.

FIG. 52 illustrates the SER and CDF of the SSI for DIDO systems with 4clients.

FIG. 53 shows the statistics of the BTSs that have been selected fortransmission and their relative usage.

DETAILED DESCRIPTION

One solution to overcome many of the above prior art limitations is anembodiment of Distributed-Input Distributed-Output (DIDO) technology.DIDO technology is described in the following patents and patentapplications, all of which are assigned the assignee of the presentpatent and are incorporated by reference. These patents and applicationsare sometimes referred to collectively herein as the “related patentsand applications.”

U.S. application Ser. No. 13/464,648, entitled “System and Methods toCompensate for Doppler Effects in Distributed-Input Distributed OutputSystems.”

U.S. application Ser. No. 12/917,257, entitled “Systems And Methods ToCoordinate Transmissions In Distributed Wireless Systems Via UserClustering”

U.S. application Ser. No. 12/802,988, entitled “Interference Management,Handoff, Power Control And Link Adaptation In Distributed-InputDistributed-Output (DIDO) Communication Systems”

U.S. Pat. No. 8,170,081, issued May 1, 2012, entitled “System And MethodFor Adjusting DIDO Interference Cancellation Based On Signal StrengthMeasurements”

U.S. application Ser. No. 12/802,974, entitled “System And Method ForManaging Inter-Cluster Handoff Of Clients Which Traverse Multiple DIDOClusters”

U.S. application Ser. No. 12/802,989, entitled “System And Method ForManaging Handoff Of A Client Between DifferentDistributed-Input-Distributed-Output (DIDO) Networks Based On DetectedVelocity Of The Client”

U.S. application Ser. No. 12/802,958, entitled “System And Method ForPower Control And Antenna Grouping In ADistributed-Input-Distributed-Output (DIDO) Network”

U.S. application Ser. No. 12/802,975, entitled “System And Method ForLink adaptation In DIDO Multicarrier Systems”

U.S. application Ser. No. 12/802,938, entitled “System And Method ForDIDO Precoding Interpolation In Multicarrier Systems”

U.S. application Ser. No. 12/630,627, entitled “System and Method ForDistributed Antenna Wireless Communications”

U.S. Pat. No. 7,599,420, issued Oct. 6, 2009, entitled “System andMethod for Distributed Input Distributed Output Wireless Communication”;

U.S. Pat. No. 7,633,994, issued Dec. 15, 2009, entitled “System andMethod for Distributed Input Distributed Output Wireless Communication”;

U.S. Pat. No. 7,636,381, issued Dec. 22, 2009, entitled “System andMethod for Distributed Input Distributed Output Wireless Communication”;

U.S. Pat. No. 8,160,121, issued Apr. 17, 2012, entitled, “System andMethod For Distributed Input-Distributed Output WirelessCommunications”;

U.S. application Ser. No. 11/256,478, entitled “System and Method ForSpatial-Multiplexed Tropospheric Scatter Communications”;

U.S. Pat. No. 7,418,053, issued Aug. 26, 2008, entitled “System andMethod for Distributed Input Distributed Output Wireless Communication”;

U.S. application Ser. No. 10/817,731, entitled “System and Method ForEnhancing Near Vertical Incidence Skywave (“NVIS”) Communication UsingSpace-Time Coding.”

To reduce the size and complexity of the present patent application, thedisclosure of some of the related patents and applications is notexplicitly set forth below. Please see the related patents andapplications for a full detailed description of the disclosure.

1. System Model

Described below is a multi-user (MU) multiple antenna system (MAS), orMU-MAS, consisting of a precoding transformation unit 101, a network 102and M transceiver stations 103 communicating wirelessly to N clientdevices UE1-UE4, as depicted in FIG. 1. The precoding transformationunit 101 receives N streams of information with different networkcontents (e.g., videos, web-pages, video games, text, voice, etc.,streamed from Web servers or other network sources C1-C5) intended fordifferent client devices. Hereafter, we use the term “stream ofinformation” to refer to any stream of data sent over the networkcontaining information that can be demodulated or decoded as astandalone stream, according to certain modulation/coding scheme orprotocol, to produce certain voice, data or video content. In oneembodiment, the stream of information is a sequence of bits carryingnetwork content that can be demodulated or decoded as a standalonestream. In one embodiment, this network content is delivered to theprecoding transformation unit 101 via a network. Any type of networkaccess technology may be used including wireline and wireless.Additionally, the network may be a local area network (e.g., LAN, WLAN,etc.), wide area network, the Internet, or any combination thereof.

In one embodiment, the precoding transformation unit 101 processes thechannel state information (CSI) for each communication channelestablished with each client device UE1-UE4 to produce a precodingtransformation. In another embodiment, channel quality information(e.g., signal-to-noise ratio, etc) or statistical channel information(e.g., spatial covariance matrix, etc.) are used to compute theprecoding transformation. The precoding transformation can be linear(e.g., zero-forcing [1], block-diagonalization [2], matrix inversion,etc.) or non-linear (e.g., dirty-paper coding [3-5] orTomlinson-Harashima precoding [6-7]).

In one embodiment, the precoding transformation unit 101 utilizes theprecoding transformation to combine (according to certain algorithm) theN streams of information from the network content C1-C5 into M streamsof bits. Hereafter, we use the term “stream of bits” to refer to anysequence of bits that does not necessarily contain any useful bit ofinformation and as such cannot be demodulated or decoded as a standalonestream to retrieve the network content. In one embodiment of theinvention, the stream of bits is the complex baseband signal produced bythe precoding transformation unit and quantized over given number ofbits to be sent to one of the M transceiver stations 103. In oneembodiment, the M streams of bits are sent from the precodingtransformation unit to the M transceiver stations 103 via the network102 (which may be a wireline/wireless, Internet, wide area network, orlocal area network, or any combination thereof).

Finally, the M transceiver stations 103 send the streams of bits to theclient devices UE1-UE4 that recover the streams of information anddemodulate the network content. Note that the number of clients K in thesystem can be any value. For example, if K>M the extra (K−M) clients aremultiplexed via different techniques described in the related patentsand applications and in the prior art (e.g., TDMA, FDMA, OFDM, CDMA,etc.). Also, if K<=M but K<N, more than one stream of information isavailable for some of the client devices. Those client devices candemodulate multiple streams of information if they are equipped withmultiple antennas by using existing MIMO or DIDO techniques.

One important feature of the present invention is that the MU-MAStransforms the streams of information into streams of bits sent over thenetwork to the transceiver stations 103, such that the client devicesUE1-UE4 can recover the stream of information when receiving the streamsof bits simultaneously from all transceiver stations. We observe that,unlike prior art, the M streams of bits sent through the network arecombinations of some or all N streams of information. As such, if aclient device had to receive the stream of bits from only one of the Mtransceiver stations (even assuming good link quality and SNR from thatstation to the client), that information would be completely useless andit would be impossible to recover the original network content. It isonly by receiving the streams of bits from all or a subset of the Mtransceiver stations that every client device can recover the streams ofinformation and demodulate the network contents C1-C5.

In one embodiment of the invention, the MU-MAS is a distributed-inputdistributed-output (DIDO) system consisting of a centralized processor(CP) 201, base transceiver stations (BTSs) 203, and user equipment (UEs)UE1-UE4 as shown in FIG. 2 and described in the related patents andapplications referenced above. The BTSs can either be directly connectedto the CP 201 as in FIG. 2 or via the base station network (BSN) 301 asdepicted in FIG. 3. In another embodiment, the network contents C1-C5and the BTSs 203 are both connected to the CP 201 via the same network401 as in FIG. 4, which may be a wireline/wireless local area network,wide area network, and/or the Internet.

For client devices to reliably recover the network content from thereceived streams of information, the wireless channel must have asufficient number of degrees of freedom or equivalently must have highspatial diversity. Spatial diversity depends on the distribution inspace of the transceiver stations 203 and the client devices UE1-UE4 aswell as the spatial distribution of multi-paths in the propagationenvironment (or channel angular spread). Described below are differentmetrics to evaluate the spatial diversity of the wireless channel thatwill be used in the techniques and methods described later on in thepresent application.

2. Diversity Metrics and Channel Models

The received signal at target client k is given by

$\begin{matrix}{r_{k} = {{H_{k}W_{k}s_{k}} + {H_{k}{\sum\limits_{\underset{u\; \neq \; k}{u = 1}}^{K}{W_{u}s_{u}}}} + n_{k}}} & (1)\end{matrix}$where k=1, . . . , K, with K being the number of clients. Moreover,r_(k)∈C^(R×M) is the vector containing the receive data streams atclient k, assuming M transmit DIDO antennas and R receive antennas atthe client devices; s_(k)∈C^(N×1) is the vector of transmit data streamsto client k in the main DIDO cluster; s_(u)∈C^(N×1) is the vector oftransmit data streams to client u in the main DIDO cluster;n_(k)∈C^(N×1) is the vector of additive white Gaussian noise (AWGN) atthe R receive antennas of client k; H_(k)∈C^(R×M) is the DIDO channelmatrix from the M transmit DIDO antennas to the R receive antennas atclient k; W_(k)∈C^(M×R) is the matrix of DIDO precoding weights toclient k in the main DIDO cluster; W_(u)∈C^(M×R) is the matrix of DIDOprecoding weights to client u in the main DIDO cluster.

To simplify the notation without loss of generality, we assume allclients are equipped with R receive antennas and that there are M DIDOdistributed antennas with M≥(R·K). If M is larger than the total numberof receive antennas, the extra transmit antennas are used to pre-cancelinterference to the target clients or to improve link robustness to theclients within the same cluster via diversity schemes described in therelated patents and applications, including U.S. Pat. Nos. 7,599,420;7,633,994; 7,636,381; and application Ser. No. 12/143,503.

The DIDO precoding weights are computed to pre-cancel inter-clientinterference. For example, block diagonalization (BD) precodingdescribed in the related patents and applications, including U.S. Pat.Nos. 7,599,420; 7,633,994; 7,636,381; and application Ser. No.12/143,503 and [2] can be used to remove inter-client interference, suchthat the following condition is satisfied in the main clusterH_(k)W_(u)=0^(R×R); ∀u=1, . . . , K; with u≠k.  (2)Substituting conditions (2) into (1), we obtain the received datastreams for target client k, where inter-user interference is removedr _(k) =H _(k) W _(k) s _(k) +n _(k).  (3)We define the effective channel matrix of user k as{tilde over (H)}_(k)=H_(k)W_(k).  (4)

One embodiment of the invention defines the diversity metric as theminimum over all clients of the minimum singular values of the effectivechannel matrices in (4)

$\begin{matrix}{\lambda_{\min} = {\min\limits_{{k\; = \; 1},\;\ldots\;,\; K}{{\lambda_{\min}^{(k)}\left( {\overset{\sim}{H}}_{k} \right)}.}}} & (5)\end{matrix}$

Another embodiment uses the minimum or maximum singular value or thecondition number of the composite DIDO channel matrix obtained bystaking the channel matrices from every client as

$\begin{matrix}{\overset{\_}{H} = {\begin{bmatrix}H_{1} \\\vdots \\H_{K}\end{bmatrix}.}} & (6)\end{matrix}$The condition number (CN) is defined as the ratio between the maximumand the minimum singular value of the composite DIDO channel matrix as

$\begin{matrix}{{C\; N} = {\frac{\lambda_{\max}\left( \overset{\_}{H} \right)}{\lambda_{\min}\left( \overset{\_}{H} \right)}.}} & (7)\end{matrix}$

Next, we define different channel models that will be used to simulatethe performance of the system and methods described in this applicationin realistic propagation conditions. We employ the well known Kroneckerstructure [8,9] and model the spatial covariance matrix withcross-correlation entries r_(i,j)=ρ_(c) ^(|i−j|) with i≠j, andauto-correlation entries given by

$\begin{matrix}{r_{i,i} = \left\{ {\begin{matrix}\rho_{a} & {i = 1} \\\frac{\left( {N - \rho_{a}} \right)\left( {N - i + 1} \right)}{\sum\limits_{n = 1}^{N - 1}n} & {i > 1}\end{matrix}.} \right.} & (8)\end{matrix}$The advantage of the model in (8) is that it allows us to write thediagonal entries of the spatial covariance matrix as a function of onlyone parameter ρ_(a).

We define three different channel models for the transmit spatialcovariance matrix: i) “i.i.d. model” with ρ_(c)=0.00001, ρ_(a)=1 thatapproximates the independent identically distributed models; ii) “highcross-correlation model” with ρ_(c)=0.8, ρ_(a)=1 to simulate wirelesssystems where the antennas have equal transmit power and are in closeproximity to each other (e.g., corner case in MIMO systems) therebyyielding high cross-correlation coefficients; iii) “highauto-correlation model” with ρ_(c)=0.00001, ρ_(a)=5.9 to simulatewireless systems with antennas distributed over a large area to yieldlow spatial correlation, but with one antenna overpowering all theothers due to its close proximity to all clients (e.g., corner case inDIDO systems). Simulated transmit covariance matrices for DIDO 6×6systems with these three models are shown in FIG. 5. In all the resultspresented hereafter, we assume the receive covariance matrix isidentity, since the clients are assumed to be spread over large area,several wavelengths apart from one another.

FIG. 6 shows the cumulative density function (CDF) of the diversitymetric (i.e., minimum singular value) as well as the symbol error rate(SER) performance as a function of the signal-to-noise ratio (SNR) forthe three channel models described above. We observe the SER performancein the “high cross-correlation model” and “high auto-correlation model”degrades due to lack of spatial diversity. In the “highcross-correlation model”, lack of diversity is due to high spatialcorrelation across the transmit antennas of the MIMO array. In the “highauto-correlation model”, reduced diversity is due to transmit powerimbalance on one of the transmit antennas over the others. One way toimprove spatial diversity in the “high cross-correlation model” is tospace antennas far apart, which can be prohibitive in practical MIMOsystems. In the “high auto-correlation model” diversity can be increasedby balancing transmit power across the distributed antennas as describedlater in this patent application.

We collected the diversity metric in DIDO systems for a variety ofpropagation conditions. In the experimental campaign, we used the DIDOBTSs installed in different buildings in downtown Palo Alto, as shown inFIG. 7. We began by measuring the “space selectivity indicator” (SSI) in(5) for DIDO 2×2 systems in a variety of propagation scenarios and withdifferent combinations of transmit BTSs and receive antenna locationsaround downtown Palo Alto.

FIG. 8 shows the SSI as a function of the maximum auto-correlation andcross-correlation coefficients of the spatial covariance matrix. Eachdot is obtained by averaging the spatial covariance matrix over a periodof 5 seconds, which is enough to average out the fading effect at thespeed considered in the experiments (i.e., stationary clients as well asclients moving at 3 mph). We observe the highest values of SSI (thatindicate high channel spatial selectivity) are obtained when the maximumcross-correlation is “0” (i.e., low spatial correlation between transmitantennas, due to large physical spacing between antennas or high channelangular spread) and the maximum auto-correlation is “1” (i.e., goodpower balance across transmit antennas). Any scenario that deviates fromthese two cases yields low values of SSI and low spatial diversity.

FIGS. 9a-9b shows the SSI results for the three channel models describedabove. The “i.i.d.” case in FIG. 9a indicates scenarios where the DIDOBTSs were physically faraway from one another; the “highcross-correlation” case was obtained by spacing the transmit antennasone wavelength apart while allowing the clients to move around anywherein the coverage area; the “high auto-correlation” case was obtained byplacing all clients in proximity of antenna 10 in FIG. 7, such that itwould overpower the other transmitter. The pink circle in every plotindicates the average of the auto- and cross-correlation coefficients.In FIG. 9b the average cross-correlation increases from FIG. 9a due tosmall antenna spacing and the average auto-correlation decreases due totransmit power balance. Vice versa, in FIG. 9c the averageauto-correlation increases due to transmit power imbalance and theaverage cross-correlation decreases due to larger antennas spacingbetween BTSs.

FIG. 10 compares the cumulative density function (CDF) of the SSI in thethree scenarios above. We observe that the i.i.d. case yields the bestperformance over the other two channel conditions.

Next, we analyze how the signal-to-noise-plus-distortion ratio (SNDR)and SER performance of DIDO systems varies as a function of spatial andtemporal variations. The spatial variations are measured via the abovedefined SSI. The temporal variations are measured through the “timeselectivity indicator” (TSI). One embodiment of the inventions definesthe TSI as the absolute value of the sum of the complex channel gainfrom some or all transmit antennas in the DIDO system. Any other metrictracking channel variations, deep-fade rate or duration can be used asTSI. The top row in FIG. 11 shows the SNDR of both client devices in theDIDO 2×2 system for one measurement set. The second row depicts the TSI:we observe the TSI of client 1 is flat because it is stationary, whereasthe TSI of client 2 fades over time due to client's mobility. The thirdrow shows the SSI as well as the auto-correlation coefficients for eachof the two transmitters. We observe that when the two auto-correlationcoefficients are close to “1”, the SSI increases.

FIG. 12 combines the above results all on one plot. For the firstreceiver (i.e., RX1) we observe the SNDR trace fades even though theclient is stationary and the TSI is flat. In fact, due to the mobilityof RX2, the SSI varies over time and those variations produce SNDR fadesalso for RX1. Moreover, we observe that the SSI may fade independentlyon the TSI. In fact TSI fades indicate poor signal quality from bothtransmitters due to destructive interference of multipaths, but the DIDOchannel may still have enough spatial degrees of freedom (i.e., largeSSI) to support multiple spatial data streams. Alternatively, SSI fadesindicate the DIDO channel matrix is singular and cannot support multipleparallel data streams, but the signal quality from all transmit antennasmay still be good, yielding large TSI. The present invention uses TSI,SSI, or a combination of the two metrics to evaluate the channeldiversity and adjust system parameters to enhance diversity.

The SSI can be used to measure and predict the areas of coherence inDIDO systems. For example, one embodiment of the invention measures theSSI, keeps track of it over time, and predicts its future behavior.Based on that prediction, it adapts both transmit and receive systemparameters (e.g., number of BTSs to employ for transmission or number ofclient devices to receive data streams).

FIG. 13 shows the results from a second channel scenario where RX2 movesfrom a location with high power imbalance to another with low powerimbalance. The result of this variation is SSI improvement. FIG. 14shows the summary for the same measurement scenario: we observe even theperformance of RX1 (the stationary client) improves as the SSIincreases.

We compared the SNDR performance against the SSI in a large set ofpropagation conditions. FIGS. 15a-15b depicts the SNDR versus theaverage SSI in a) and standard deviation of the SSI in b). Each dotrepresents one measurement collected over a period of 5 seconds. The redsolid line represents the linear regression of all measurement points,where the upper and lower 10% of the measurements has been removed fromthe regression calculation. We observe the average SNDR increases as afunction of the average SSI in FIG. 15a due to large spatial diversityavailable in the wireless channel. For example, scenarios characterizedby large power imbalance yield low average SSI resulting in low SNDR.Moreover, in FIG. 15b the average SNDR decreases as a function of thestandard deviation of the SSI due to frequent deep-fades of the SSI dueto client mobility. Note that in practical systems the average andstandard deviation of the SSI can be computed via running averagetechniques or methods using forgetting factor for efficient memory useand reduction in computational complexity.

FIG. 16 shows similar results for the temporal channel variations. TheSNDR decreases as the standard deviation of the TSI due to deep-fade intime caused by the client's mobility and Doppler effect.

Another embodiment of the invention uses the condition number (CN) asSSI. The CN defined in equation (7) is plotted as a function of theminimum auto-correlation coefficient and maximum cross-correlationcoefficient in FIG. 17b , where every dot is the result of 100 msec ofdata. Contrarily to the minimum singular value of the effective channelmatrix, lower CN indicates channels with high spatial diversity. FIG.17a shows the CDF of the SSI defined as the CN. FIG. 18 depicts thethree dimensional version of FIG. 17 a.

FIG. 19 shows the average SNDR as a function of the average CN. Everydot represents an average over 20 seconds of data. We observe that theSNDR degrades as the value of the average CN increases.

3. Methods for Transmit Antenna Selection

One way to increase the spatial degrees of freedom in a wireless link isto add more transmit antennas than the number of clients in the systemand select among the antennas that satisfy a certain SSI performancetarget. This algorithm is known as transmit antenna selection asdescribed in [10] and our previous patent application U.S. Pat. No.7,636,381. In one embodiment, all possible combination of transmitantenna subsets are first identified. Then the SSI is computed for eachof the antenna sets. Finally the set that maximizes the diversity metricor SSI is chosen as optimal transmit antenna subset.

FIGS. 20a-20b show the performance of different order DIDO systems ini.i.d. channels assuming same per-antenna transmit power. The SSIdegrades for increasing number of transmit antennas as the CDF shifts tothe left going from 2×2 to 8×8, but the SER performance is similar forany order DIDO.

FIGS. 21a-21b show the simulated performance of DIDO 4×4 in i.i.d.channels. Antenna selection provides significant gains in SNR dependingon the target SER. For example, at SER target of 1% by adding two extraantennas the gain is 12 dB or at a target of 0.1% that gain increases upto 18 dB. Also, FIG. 21b shows that the CDF of the SSI improves withantenna selection due to enhanced spatial diversity. Note that in FIG.21 we plot the maximum (over all possible transmit antenna subsets) ofthe minimum singular value of the effective channel matrix. We definethe mean value of the CDF in FIG. 21b in decibels asD _(dB)=20 log₁₀(E{max(λ_(min))})  (9)

FIGS. 22a-22b show the gain in SNR as a function of the gain of thediversity metric in (9). The values in the table a) are obtained fromthe simulated SER performance in FIG. 21a . In FIG. 22b we observe closeto linear relation between the two gains. In one embodiment of theinvention, the average SSI is used to decide whether to employ theselection algorithm or not. In fact, antenna selection algorithmsrequire additional computational complexity as the SSI must be computedover all antenna subsets. Understanding under what channel conditionsthe antenna selection algorithm is really needed, allows to turn off thealgorithm when unnecessary, thereby improving computational complexityof the system. For example, if the average SSI is above a certainthreshold, there is no need to trigger the antenna selection algorithmand a fixed number of antennas are used for transmission. If the averageSSI decreases (e.g., due to client's mobility) the algorithm willtrigger the antenna selection algorithm in such a way that the bestantenna subset is chosen for transmission over the wireless link.

The SSI threshold can be pre-calculated by analyzing experimental datafrom practical measurements. For example, FIGS. 23a-23b show the tracesof SNDR, SSI (indicated as λ_(min)) and TSI (being the absolute value ofthe complex channel gain from every transmitter indicated as TX1, . . ., M). Note that we intentionally subtracted 20 dB from the TSI traces tofit all traces into the same plot while avoiding overlaps. In thisexperiment, the first client RX1 is stationary, whereas the second RX2is mobile. We observe that even for the stationary client the SNDR tracevaries over time due to fades in the SSI trace. In particular, everytime the SSI falls below −10 dB, the SNDR undergoes deep-fades. Wechoose −10 dB as a threshold for the SSI. This invention is not limitedto this value of SSI and other values may be chosen based on differentperformance criteria. For the mobile client, deep-fades are caused byeither SSI fades or TSI fades. As observed before, these two types offades are uncorrelated and may occur at different times.

FIGS. 24a-24b show the results obtained over the same route as in FIGS.23a-23b , but using two extra antennas. Note that the SNDR trace is notaligned with the one in FIGS. 24a-24b because the instantaneous channelvaries from one experiment to the next due to fast-fading effects. Weobserve that, by adding two extra antennas and running the transmitantenna selection algorithm, it is possible to remove deep-fades fromthe SSI trace and improve SNDR performance of both clients. FIG. 24ashows that the stationary client does not undergo any SNDR deep fade.FIG. 24b shows that the SNDR fades of the mobile client are only due toTSI, whereas the SSI fades are completely removed.

One embodiment of the invention scans through the available transmitantenna subsets until the first one that provides SSI above thepredefined threshold is reached. Once that subset is found, the searchstops thereby reducing the computational complexity of the algorithm.

In FIGS. 23a-23b and 24a-24b we observed that the SSI exhibits astructured behavior with periods of deep-fade that alternate to periodsof high gain. This temporal correlation between consecutive samples ofSSI can be exploited to reduce the complexity of the antenna subsetselection algorithm. In one embodiment, the same antenna subset ispreserved until the SSI undergoes a deep-fade and its value drops belowthe predefined threshold. In another embodiment, the system keeps trackof the statistics of the antenna subset that have been chosen in thepast and only selects those for future transmissions.

Another way to reduce computational complexity of the system is toreduce the number of combinations of transmit antennas to be chosenacross with the antenna selection method. FIGS. 25a-25b, 26a-26b,27a-27b and 28a-28b show the simulated SER performance and CDF of theSSI for DIDO 4×2, 6×4, 8×6 and 10×8 systems. All these systems employtwo extra antennas than the number of clients. Performance is shown fordifferent numbers Ns of antenna subsets. We observe that, for any DIDOorder, 10 or less subsets of transmit antennas suffice to approximateclosely the SER performance of the same system using all possiblecombinations of antenna subsets. Reducing the number of antenna subsetscan yield a significant reduction in computational complexity as the SSIdoes not need to be computed over all antenna subsets. One embodiment ofthe invention selects a limited number of subsets as a means to reducethe computational complexity of the system, while maintaining systemperformance close to ideal performance.

One embodiment of the invention uses combination of SSI and TSI toselect the optimal antenna subset. For example, the antenna subset thatprovides the maximum SSI and TSI is selected. Another embodiment definesa first selection phase that identifies all antenna subsets that provideSSI above the predefined threshold. Then, a second selection phasechooses the subset that yields the largest TSI. Alternatively, anotherthreshold is defined for the TSI and the subset that satisfies both SSIand TSI thresholds is selected.

All the methods and results described above for single-carrier systemscan be directly extended to multi-carrier and/or OFDM systems bydefining “frequency selectivity indicator” (FSI). For example, in OFDMsystems every tone experiences a frequency flat channel. Then allmethods described above can be applied on a tone-by-tone basis. Inanother embodiment, different combinations of SSI, TSI and FSI areemployed to select the optimal antenna subset according to the criteriadefined above.

Finally, we show the performance of antenna selection algorithms in avariety of propagation conditions. FIGS. 29a-29b depict the average SNDR(over periods of 5 seconds) versus the average SSI. Large average SSIindicates channels with high spatial diversity, thereby yielding largeaverage SNDR. We observe that two extra antennas used for antennaselection going from DIDO 2×2 to 4×2 increases both average SNDR andSSI. Similar results are shown in FIGS. 30a-30b for DIDO 4×4 and 6×4.

FIGS. 31a-31b depicts the average SNDR as a function of the standarddeviation of the TSI for DIDO 2×2 and 4×2. High standard deviationindicates large time selectivity due to Doppler effects that degrade theSNDR performance. Similar results are showed in FIGS. 32a-32b for DIDO4×4 and 6×4.

Finally, FIG. 33 compares the CDF of the instantaneous SSI for all fourDIDO systems considered above. We observe that 4×4 has worse CDFperformance than 2×2 due to reduced degrees of freedom when switching tohigher order DIDO. In both cases, adding 2 extra antennas with transmitselection algorithms yield significant improvement in SSI performance.

4. Methods for User Selection

In one embodiment, spatial diversity is enhanced in DIDO channels viauser selection. In this embodiment, if there are not enough degrees offreedom in the wireless channel for the given number of transmitantennas available in the system, then the system drops transmission toone or multiple clients. This technique may employ the SSI to measurethe spatial diversity in the wireless link. When the SSI falls below apredefined threshold, one or multiple clients are dropped.

In one embodiment of the invention, the fastest moving client isdropped. In fact, the client experiencing the highest Doppler effect ismost likely to undergo deep-fades. Another embodiment utilizes the TSIand FSI to select the client with lower channel quality and drops thatclient. When the client is dropped, the bits transmitted over thatperiod are corrupted and those bits can be recovered via forward errorcorrection (FEC) coding. Another embodiment utilizes alternativemultiplexing technique such as TDMA, FDMA, OFDMA or CDMA to serve thedropped clients.

FIG. 34 shows the results from one particular measurement set withapproximately 20 seconds of data. The first row depicts the measuredSNDR trace for the two clients denoted as RX1 (stationary client) andRX2 (mobile client); the second row is the simulated SNDR with thetarget fixed to 10 dB to demodulate 4-QAM constellations reliably; thethird row is the simulated SER; finally the fourth row depicts the SSIand the auto-correlation coefficients. We observe that even if RX1 isstationary, its SNDR drops below the target due to lack of spatialdiversity, as indicated by low SSI. If the SSI target is too low (i.e.,−20 dB in the figure) the user selection algorithm is never triggeredand RX1 undergoes severe SER degradation.

FIG. 35 shows the case where the SSI target is raised to −15 dB and as aresult the SER performance improves. Further improvement is achieved byraising the target SSI further up −10 dB as in FIG. 36, in which casethe SER for RX1 is reduced to zero throughout the duration of themeasurement. In this case the SSI threshold is determined based on theSER performance, but this invention is not limited to that and any otherperformance criterion can be used for that.

5. Methods for Transmit Power Balancing

Transmit power imbalance occurs when most or all of the clients arearound one BTS and far from all the others, such that one BTS overpowersthe others. Transmit power imbalance reduces channel spatial diversity(i.e., decreases the SSI), thereby adversely affecting systemperformance. One exemplary scenario is shown in FIG. 37 where multipleUEs 3701 (identified as squares) are clustered around one particular BTS3702 (identified with a circle) and located far away from the otherBTSs. This scenario would happen, for example, when there is an event inone location in which the group of clients are participating, and allother BTSs are far away. One embodiment of the invention adaptivelyadjusts the power of the BTSs in such a way that the power received atall clients from all BTSs is balanced. In one embodiment of theinvention, the power of the BTS that is overpowering all the others isreduced until the power received by the clients balances the powerreceived from all other BTSs. In another embodiment of the invention thepower from all other BTSs is increased until the received power levelfrom all BTSs to every client is balanced.

In TDD systems in which channel reciprocity is exploited, the channelstate information (CSI) for the downlink is obtained from the uplink.The uplink training signal is quantized by the ADC at the receiver ofthe BTS and, as such, it has limited dynamic range, depending on thenumber of bits of the ADC. If all clients are clustered around one ofthe BTSs, the CSI for that BTS will have a much larger amplitude thanthe one from all the others and, as such, it will make the DIDO channelmatrix singular and limit the spatial degrees of freedom of the link.That is the effect of transmit power imbalance. In FDD systems or TDDsystems that do not exploit channel reciprocity, the same issuemanifests at the receiver of the client devices also equipped with ADC.Moreover, the CSI may need to be quantized or mapped into bits vialimited feedback techniques, before being sent over the wireless link.That quantization again limits the dynamic range for the CSI and yieldsa power imbalance when one of the BTSs overpowers the other. Embodimentof the invention described herein employ techniques for preventing powerimbalance in MU-MAS and DIDO systems.

As shown in FIG. 13, one way to identify transmit power imbalance is bylooking at the auto-correlation coefficient ρ_(a): when theauto-correlation value approximates the number of the BTSs (assuming thetransmit spatial covariance matrix is normalized with trace equal to thenumber of BTSs) the system undergoes transmit power imbalance. Forexample, in a power imbalanced DIDO 4>4 system, one auto-correlationwould be close to “4” and all other auto-correlation coefficients wouldbe close to zero. Contrarily, in a perfectly balanced system, allauto-correlation coefficients would be “1”.

Transmit power imbalance adversely affects the performance of thesystem. For example, FIG. 38 shows the SER performance of differentorder DIDO systems for different values of the maximum auto-correlationcoefficient. As the maximum auto-correlation decreases to “1”, the SERperformance approaches the ideal i.i.d. case. These SER results can beused to define thresholds that distinguish balanced systems fromimbalanced systems. These auto-correlation thresholds can be determinedthrough numerical, analytical, or empirical methods. For example, inFIG. 38 the thresholds are chosen such that the SER performance does notdegrade more than 3 dB from the ideal i.i.d. performance. The invention,however, is not limited to this performance criterion and any othercriteria that measure the system performance can be used. Anotherembodiment of the invention employs a hysteresis loop where twodifferent thresholds are defined for the auto-correlation coefficient asin the table in FIG. 38.

FIG. 39 shows the CDF of the SSI for different values of maximumauto-correlation. We observe that increasing the maximumauto-correlation yields worse SSI performance due to reduced spatialdiversity.

Embodiments of the invention propose different methods for balancing thetransmit power across all BTSs in the MU-MAS or DIDO system. Thesemethods can be executed at a regular rate. In one embodiment, theproposed methods run every execution cycle. However, depending on theconstraints of the system being used, a lower rate may be used.Hereafter, we described these methods in details.

One embodiment of the invention aims to keep the transmit power of eachBTS at the maximum possible level, while staying within theauto-correlation thresholds. We define two different thresholds, asshown in FIG. 38. The upper threshold, MAX_AUTO_CORR, represents thepoint at which the auto correlation number results in a significant dropin spatial diversity. If the auto-correlation number goes above thisthreshold, there will be a large drop in system performance.

The lower threshold, MIN_AUTO_CORR acts as a buffer to prevent thesystem from changing power settings too often. If a given BTS has anauto correlation number below MIN_AUTO_CORR, it can safely increase itstransmit gain value (assuming transmit gain is not already set to itsmaximum). Note that the transmit gain may be the analog gain of thepower amplifier in the RF chain and/or the digital gain corresponding toa certain level of the DAC. If the auto-correlation is between theMIN_AUTO_CORR and MAX_AUTO_CORR, no action is taken. If the power was tobe increased in this instance, it could increase the auto-correlationnumber until it was above the MAX_AUTO_CORR, at which point the powerwould be decreased until it was below the MAX_AUTO_CORR, etc. Thiseffect would cause the power to be changing constantly, which isinefficient and may potentially cause performance degradation.

One embodiment of a method is illustrated in FIG. 40 and its associatedpseudo-code is described as follows:

BEGIN INITIALIZE txGain for each BTS SET highestAutoCorrNum = 0 SET K =0 REPEAT WHILE K < number of BTSs IF auto correlation number for BTS K >highestAutoCorrNum THEN SET maxAutoCorrNum = auto correlation number ofBTS K SET N = K END IF INCREMENT K END REPEAT IF highestAutoCorrNum >MAX_AUTO_CORR AND txGain for BTS N > MIN_TX_GAIN THEN Decrease thetxGain for BTS N by TX_GAIN_STEP Set radio TX gain for BTS N = txGainfor BTS N SET K = 0 REPEAT WHILE K < number of BTSs IF txGain for BTS K< MAX_TX_GAIN AND auto correlation number for BTS K < MIN_AUTO_CORR THENINCREASE txGain for BTS K by TX_GAIN_STEP SET radio TX gain for BTS K totxGain for BTS K END IF INCREMENT K END REPEAT END

In summary, this method first determines which BTS has the highestcorrelation. That correlation value is saved, along with the index ofthe corresponding BTS. Then, if the highest correlation is above theupper threshold, the transmit gain is decreased. The transmit gain willnot decrease below a defined minimum. Then, for each BTS, the transmitgain is increased if the highest correlation is below the lowest value.If the highest auto-correlation number is between the two thresholds, noaction is taken. This is the target mode of operation of the proposedmethod.

Turning to the specific details of FIG. 40, at 4001, the variableshighestAutoCorrNumber and K are initialized to zero. Steps 4002 and 4004ensure that the loop repeats for each BTS. At 4002, if the current valueof K is currently less than the number of BTSs, then at 4003, adetermination is made as to whether the autocorrelation number for BTS Kis greater than the current highest auto-correlation number. If so, thenat 4005 the variable highestAutoCorrNum is set to the auto-correlationnumber for BTS K (i.e., BTS K has the highest auto-correlation number)and control variable N is set equal to K.

At 4006, if the highestAutoCorrNum is greater than the maximumauto-correlation (MAX_AUTO_CORR) and the transmit gain (txGain) for BTSN is greater than the minimum transmit gain (MIN_TX_GAIN) then, at 4008,the transmit gain for BTS N is decreased using a specified step size(TX_GAIN_STEP) and the txGain of BTS N's radio is set to the new txGainvalue.

At 4009, the control value K is set equal to zero. Step 4010 ensuresthat each BTS is addressed by the loop of steps 4011-4012. That is, if Kis currently less than the number of BTSs (i.e., if all BTSs have notbeen analyzed) then, at 4011, a determination is made as to whether theauto-correlation number for BTS K is less than the minimumauto-correlation (MIN AUTO CORR) and the txGain for BTS K is less thanthe maximum allowable transmit gain value (MAX_TX_GAIN). If bothconditions are met then, at 4012, the transmit gain for BTS K isincreased by the predefined step size (TX_GAIN_STEP) and the new txGainis set on BTS K's radio. The control value K is incremented at 4013 and,at 4010, if K is equal to the number of BTSs (i.e., each BTS has beenanalyzed), the process terminates.

In another embodiment of the invention, auto-correlation values aremapped to transmit gain values. One embodiment uses a linear mapping,shown below. Although a linear mapping is simple to implement, theadverse effect of the auto-correlation on system performance does notscale linearly. Typically, system performance is significantly affectedonly after the auto-correlation number reaches some fraction of itsmaximum value. For example, DIDO 2×2 performance is seriously affectedonly when the maximum auto-correlation is above 1.95 (or 97.5% of itsmaximum value). Another mapping algorithm may utilize an exponentialfunction or another power function designed to operate in these ranges,rather than a linear function.

One embodiment of the method is illustrated in FIG. 41 and itspseudo-code is described as follows:

BEGIN INITIALIZE txGain for each BTS SET K = 0 REPEAT WHILE K < numberof BTSs SET autoCorr = auto correlation number for BTS K SET txGain forBTS K = (MAX_TX_GAIN − MIN_TX_GAIN) * (1 − autoCorr/nTX) + MIN_TX_GAININCREMENT K END REPEAT

This method takes an auto-correlation number and scales it directly intoa transmit gain value. Most of the complexity in the method is to allowdifferent orders of DIDO and different values of MIN_TX_GAIN andMAX_TX_GAIN. For example, the simplest form of the equation for a DIDO2×2 system with transmit gain that ranges between A and B would be:

$\begin{matrix}{\left( {B - A} \right)*\left( {1 - \frac{\rho_{a}}{2}} \right)} & (9)\end{matrix}$

For example, an auto-correlation value of 2 (highest value for DIDO 2×2)would result in the transmit gain for that BTS being set to A=0 (lowesttransmit power), while an auto correlation value of 0 (lowest value forDIDO 2×2) would result in the transmit gain for that BTS being set toB=30 (highest transmit power). It should be noted that both of thesecases indicated extreme power imbalance. In the first case (ρ_(a)=2.0),this BTS is being received too strongly across the UEs. In the secondcase (ρ_(a)=0.0), the other BTS is being received too strongly. Aperfectly balanced system, with ρ_(a)=1.0 for both BTSs, would result inthe transmit gain staying at 15 (being the default value), as desired.

Turning to the specifics of FIG. 41, at 4101, the control variable K isinitialized to 0. At 4102, if K is less than the number of BTSs beingobserved, then the operations set forth in 4103 are performed.Specifically, the variable autoCorr is set equal to the current autocorrelation number for BTS K. In addition, the variable txGain for BTS Kis set equal to the difference between the maximum transmit gain valueand the minimum transmit gain value (MAX_TX_GAIN−MIN_TX_GAIN) multipliedby (1−autoCorr/nTX) and added to the minimum transmit gain value(MIN_TX_GAIN). Control variable K is then incremented until K=the numberof BTSs. The process then terminates.

Both of the previous methods are designed to adjust the transmit gain ofevery BTS within a single step. Another embodiment of the inventiondefines a method that always adjusts the power of only two BTSs. Withthis method, however, in certain scenarios one or more of the BTSs couldremain at low transmit power setting for long periods of time. Thus, inpractical systems this method would be combined with an algorithmsimilar to Method 1 (using thresholds as in FIG. 40) where the power ofeach BTS is increased if the auto-correlation number for that BTS isbelow MIN_AUTO_CORR.

The pseudo-code for Method 3 described above is as follows:

BEGIN INITIALIZE txGain for each BTS SET highestAutoCorrNum = 0 SETlowestAutoCorrNum = MAX_AUTO_CORR SET K = 0 REPEAT WHILE K < number ofBTS IF auto correlation number for BTS K > highestAutoCorrNum THEN SEThighestAutoCorrNum = auto correlation number of BTS K SET highestIdx = KELSE IF auto correlation number for BTS K < lowestAutoCorrNum THEN SETlowestAutoCorrNum = auto correlation number of BTS K SET lowestIdx = KEND IF INCREMENT K END REPEAT DECREASE txGain for BTS highestIdx byTX_GAIN_STEP IF txGain for BTS highestIdx < MIN_TX_GAIN THEN SET txGainfor BTS highestIdx = MIN_TX_GAIN END IF SET radio TX gain for BTShighestIdx = txGain for BTS highestIdx INCREASE txGain for BTS lowestIdxby TX_GAIN_STEP IF txGain for BTS lowestIdx > MAX_TX_GAIN THEN SETtxGain for BTS lowestIdx = MAX_TX_GAIN END IF END

In summary, this method first determines the maximum and minimumauto-correlation values and records the indices for the correspondingBTS. Then, the transmit gain of the BTS with the highest autocorrelation is reduced by TX_GAIN_STEP, and the transmit gain of the BTSwith the lowest auto correlation is increased by TX_GAIN_STEP.

Finally, we show the performance of the transmit power balancing methodsin practical outdoor propagation scenarios. The first scenario weconsidered is depicted in FIG. 42. Transmit power imbalance is caused bythe two clients UE00, UE01 being in close proximity to BTS 10. Thedistribution of the condition number with and without power imbalance isshown in FIG. 43.

FIGS. 44-46 show the channel traces (SNDR, SSI and TSI) for threedifferent cases: i) DIDO 2×2 without transmit power balancing method;ii) DIDO 2×2 with transmit power balancing method; iii) DIDO 4×2 withtransmit power balancing method in combination with antenna selectionmethod. The SSI threshold is set to −10 dB. We observe that transmitpower balancing and antenna selection methods help improve the SSI traceand consequently the SNDR performance.

A different scenario is depicted in FIG. 47, where BTS 6 is the sourceof transmit power imbalance. FIG. 48 shows the condition numberdistribution with and without imbalance and FIGS. 49-51 depict channeltraces for different algorithms as before. Once again, both transmitpower balancing and antenna selection methods improve SSI and SNDRperformance.

Another embodiment of the invention employs a combination of transmitpower balancing and antenna selection algorithms. In this method, theextra antenna that provides the largest auto-correlation coefficient isremoved and the conventional antenna selection algorithm is applied withthe remaining extra antennas. For example, FIG. 52 shows the SER and CDFof the SSI for DIDO systems with 4 clients. The performance of DIDO 6×4is significantly degraded when the system undergoes transmit powerimbalance. By removing the BTS that yields a large auto-correlationcoefficient, system performance is improved significantly as shown bythe SER curve for DIDO 5×4.

Finally, FIG. 53 shows the statistics of the BTSs that have beenselected for transmission and their relative usage. The last graph showsthat DIDO 5×4 with transmit power balancing and antenna selection haszero usage of BTS1 because that BTS has been removed as a result of highauto-correlation value.

6. References

-   [1] R. A. Monziano and T. W. Miller, Introduction to Adaptive    Arrays, New York: Wiley, 1980.-   [2] K. K. Wong, R. D. Murch, and K. B. Letaief, “A joint channel    diagonalization for multiuser MIMO antenna systems,” IEEE Trans.    Wireless Comm., vol. 2, pp. 773-786, July 2003;-   [3] M. Costa, “Writing on dirty paper,” IEEE Transactions on    Information Theory, Vol. 29, No. 3, Page(s): 439-441, May 1983.-   [4] U. Erez, S. Shamai (Shitz), and R. Zamir, “Capacity and    lattice-strategies for cancelling known interference,” Proceedings    of International Symposium on Information Theory, Honolulu, Hi.,    November 2000.-   [5] G. Caire and S. Shamai, “On the achievable throughput of a    multiantenna Gaussian broadcast channel,” IEEE Trans. Info. Th.,    vol. 49, pp. 1691-1706, July 2003.-   [6] M. Tomlinson, “New automatic equalizer employing modulo    arithmetic,” Electronics Letters, Page(s): 138-139, March 1971.-   [7] H. Miyakawa and H. Harashima, “A method of code conversion for    digital communication channels with intersymbol interference,”    Transactions of the Institute of Electronic-   [8] D.-S. Shiu, G. J. Foschini, M. J. Gans, and J. M. Kahn, “Fading    correlation and its effect on the capacity of multielement antenna    systems,” IEEE Trans. Comm., vol. 48, no. 3, pp. 502-513, March    2000.-   [9] J. P. Kermoal, L. Schumacher, K. I. Pedersen, P. E. Mogensen,    and F. Frederiksen, “A stochastic MIMO radio channel model with    experimental validation,” IEEE Jour. Select. Areas in Comm., vol.    20, no. 6, pp. 1211-1226, August 2002.-   [10] R. Chen, R. W. Heath, Jr., and J. G. Andrews, “Transmit    Selection Diversity for Unitary Precoded Multiuser Spatial    Multiplexing Systems with Linear Receivers,” IEEE Trans. on Signal    Processing, vol. 55, no. 3, pp. 1159-1171, March 2007.

We claim:
 1. A method implemented within a multiuser (MU) multipleantenna system (MAS) comprising: communicatively coupling one or morecentralized units to multiple distributed transceiver stations orantennas via a network, the network comprising wireline or wirelesslinks or a combination of both; employing the network as a backhaulcommunication channel; and employing transmit antenna selection, userselection, or transmit power balancing to enhance channel spacediversity within the MU-MAS; receiving wireless power concurrently by aplurality of user devices from the multiple antennas.
 2. The method asin claim 1 wherein a space selectivity indicator (SSI), time selectivityindicator (TSI), and/or frequency selectivity indicator (FSI) are usedas measures of the channel diversity.
 3. The method as in claim 2further comprising defining different subsets of transmit antennas andselects the subset that optimizes the SSI for transmission over thewireless links.
 4. The method as in claim 3 wherein searching for theoptimal antenna subset is suspended as soon as the first subset thatsatisfies the SSI threshold is found, thereby reducing computationalcomplexity.
 5. The method as in claim 3 wherein only a limited number ofantenna subsets are selected based on certain performance criterion as ameans to reduce the computational complexity of the method.
 6. Themethod as in claim 2 wherein a selection criterion is based on SSI, TSIand/or FSI thresholds derived from measurements.
 7. The method as inclaim 2 wherein a minimum singular value of an effective channel matrixof every client device is used as SSI.
 8. The method as in claim 2wherein a minimum singular value or the condition number of a compositechannel matrix from all client devices is used as SSI.
 9. The method asin claim 2 wherein an absolute value of the sum of the complex channelgain from some or all transceiver stations is used as TSI.
 10. Themethod as in claim 2 wherein the SSI is used to measure and predict ashape in space around each of the plurality of user devices, each shapecarrying independent and simultaneous non-interfering wireless power forthe user device.
 11. The method as in claim 2 wherein an average SSI isused to select adaptively between fixed transmit antenna configurationand transmit antenna selection methods based on changing channelconditions.
 12. The method as in claim 2 wherein temporal correlation ofthe SSI is exploited to select an optimal antenna subset while reducingthe computational complexity of the method.
 13. The method as in claim 1wherein a base transceiver station (BTS) that overpowers the other BTSsreduces its transmit power to balance the power from all BTSs to theclients.
 14. The method as in claim 1 wherein a base transceiver station(BTS) that overpowers the other BTSs keeps its power level unaltered,the other BTSs increase their transmit power to balance the power fromall BTSs to the client devices.
 15. The method as in claim 1 wherein amaximum auto-correlation coefficient of the covariance matrix is used asan indication of transmit power imbalance.
 16. The method as in claim 15wherein thresholds of auto-correlation are defined to select betweenpower balanced and imbalanced methods and those thresholds are obtainedbased on certain performance criterion.
 17. The method as in claim 16wherein the selection is based on hysteresis loop and multiplethresholds of auto-correlation are defined for that hysteresis.
 18. Themethod as in claim 15 wherein the auto-correlation is mapped into atransmit gain value and that value is used to adjust the power of theBTSs.
 19. A multiuser (MU) multiple antenna system (MAS) comprising: oneor more centralized units communicatively coupled to multipledistributed transceiver stations or antennas via a network; the networkcomprising wireline or wireless links or a combination of both, employedas a backhaul communication channel; and the MU-MAS employing transmitantenna selection, user selection and/or transmit power balancing areemployed to enhance channel spatial diversity; receiving wireless powerconcurrently by a plurality of user devices from the multiple antennas.20. The system as in claim 19 wherein a space selectivity indicator(SSI), time selectivity indicator (TSI), and/or frequency selectivityindicator (FSI) are used as measures of the channel diversity.
 21. Thesystem as in claim 20 further comprising defining different subsets oftransmit antennas and selects the subset that optimizes the SSI fortransmission over the wireless links.
 22. The system as in claim 21wherein searching for the optimal antenna subset is suspended as soon asthe first subset that satisfies the SSI threshold is found, therebyreducing computational complexity.
 23. The system as in claim 21 whereinonly a limited number of antenna subsets are selected based on certainperformance criterion as a means to reduce the computational complexityof the system.
 24. The system as in claim 20 wherein a selectioncriterion is based on SSI, TSI and/or FSI thresholds derived frommeasurements.
 25. The system as in claim 20 wherein a minimum singularvalue of an effective channel matrix of every client device is used asSSI.
 26. The system as in claim 20 wherein a minimum singular value orthe condition number of a composite channel matrix from all clientdevices is used as SSI.
 27. The system as in claim 20 wherein anabsolute value of the sum of the complex channel gain from some or alltransceiver stations is used as TSI.
 28. The system as in claim 20wherein the SSI is used to measure and predict a shape in space aroundeach of the plurality of user devices, each shape carrying independentand simultaneous non-interfering wireless power for the user device. 29.The system as in claim 20 wherein an average SSI is used to selectadaptively between fixed transmit antenna configuration and transmitantenna selection systems based on changing channel conditions.
 30. Thesystem as in claim 20 wherein temporal correlation of the SSI isexploited to select an optimal antenna subset while reducing thecomputational complexity of the system.
 31. The system as in claim 19wherein a base transceiver station (BTS) that overpowers the other BTSsreduces its transmit power to balance the power from all BTSs to theclient devices.
 32. The system as in claim 19 wherein a base transceiverstation (BTS) that overpowers the other BTSs keeps its power levelunaltered, the other BTSs increase their transmit power to balance thepower from all BTSs to the clients.
 33. The system as in claim 19wherein a maximum auto-correlation coefficient of the covariance matrixis used as an indication of transmit power imbalance.
 34. The system asin claim 33 wherein thresholds of auto-correlation are defined to selectbetween power balanced and imbalanced systems and those thresholds areobtained based on certain performance criterion.
 35. The system as inclaim 34 wherein the selection is based on hysteresis loop and multiplethresholds of auto-correlation are defined for that hysteresis.
 36. Thesystem as in claim 35 wherein the auto-correlation is mapped into atransmit gain value and that value is used to adjust the power of theBTSs.